public void SetCamera( 
   IEnumerable<Layer> layers,
   bool useSelectedFeatures
)

{
  //Create a map frame and set its camera by zooming to the extent of an existing bookmark.
  //Construct on the worker thread

  //Build 2D envelope geometry
  Coordinate2D mf_ll = new Coordinate2D(6.0, 8.5);
  Coordinate2D mf_ur = new Coordinate2D(8.0, 10.5);
  Envelope mf_env = EnvelopeBuilderEx.CreateEnvelope(mf_ll, mf_ur);

  //Reference map, create MF and add to layout
  MapProjectItem mapPrjItem = Project.Current.GetItems<MapProjectItem>()
                       .FirstOrDefault(item => item.Name.Equals("Map"));
  Map mfMap = mapPrjItem.GetMap();
  Bookmark bookmark = mfMap.GetBookmarks().FirstOrDefault(
                        b => b.Name == "Great Lakes");

  MapFrame mfElm = ElementFactory.Instance.CreateMapFrameElement(
                       layout, mf_env, mfMap, "New Map Frame");

  //Zoom to bookmark
  mfElm.SetCamera(bookmark);
}

{
  //Change the extent of a map frame to the selected features multiple layers.
  //Note: Must be on QueuedTask.Run
  //Reference MapFrame

  //Reference map, layers and create layer list
  Map m = mapFrame.Map;
  FeatureLayer fl_1 = m.FindLayers("GreatLakes").First() as FeatureLayer;
  FeatureLayer fl_2 = m.FindLayers("States_WithRegions").First() as FeatureLayer;
  var layers = new[] { fl_1, fl_2 };
  //IEnumerable<Layer> layers = m.Layers;  //This creates a list of ALL layers in map.

  //Set the map frame extent to the selected features in the list of layers
  mapFrame.SetCamera(layers, true);
}

{
  //Change map frame extent to single feature with 10 percent buffer

  //Note: Must be on QueuedTask.Run

  //Reference the mapframe and its associated map
  Map m = mapFrame.Map;

  //Reference a feature layer and build a query (to return a single feature)
  FeatureLayer fl = m.FindLayers("GreatLakes").First() as FeatureLayer;
  QueryFilter qf = new QueryFilter();
  string whereClause = "NAME = 'Lake Erie'";
  qf.WhereClause = whereClause;

  //Zoom to the feature
  using (ArcGIS.Core.Data.RowCursor rowCursor = fl.Search(qf))
  {
    while (rowCursor.MoveNext())
    {
      //Get the shape from the row and set extent
      using (var feature = rowCursor.Current as ArcGIS.Core.Data.Feature)
      {
        Polygon polygon = feature.GetShape() as Polygon;
        Envelope env = polygon.Extent as Envelope;
        mapFrame.SetCamera(env);

        //Zoom out 15 percent
        Camera cam = mapFrame.Camera;
        cam.Scale = cam.Scale * 1.15;
        mapFrame.SetCamera(cam);
      }
    }
  }
}